EP3814317B1 - Method for manufacturing mma in high yields - Google Patents

Description

field of invention

The present invention relates to a process for preparing methyl methacrylate by direct oxidative esterification of methacrolein with yields that are higher than in the prior art. Methyl methacrylate is used in large quantities to produce polymers and copolymers with other polymerizable compounds. Furthermore, methyl methacrylate is an important building block for various special esters based on methacrylic acid (MAA), which can be produced by transesterification with the corresponding alcohol. This results in a great interest in manufacturing processes for this starting material that are as simple, economical and environmentally friendly as possible.

In particular, the present invention relates to an optimized processing of the reactor discharge from the oxidative esterification of methacrolein, by means of which specific by-products can be isolated and subsequently converted into alkyl methacrylates, in particular into MMA.

State of the art

Today, methyl methacrylate (MMA) is produced using a variety of processes that start from C ₂ - C ₃ - or C ₄ building blocks. In one of these processes, MMA is obtained by oxidizing isobutylene or tert-butanol with atmospheric oxygen in the gas phase over a heterogeneous catalyst to give methacrolein (MAL) and subsequent oxidative esterification reaction of methacrolein using methanol. This process, developed by ASAHI, is included in the publications U.S. 5,969,178 and U.S. 7,012,039 described. Disadvantage of this method is in particular a very high energy requirement.

Scheme 1 below shows the production of MMA by the so-called Asahi process starting from C4 (isobutene or tert-butanol), with intermediate isolation of MAL and subsequent oxidative esterification (abbreviated “DOE”) of the MAL with methanol to form MMA, with the formation of methacrylic acid (MAS) as a by-product, shown schematically.

In a further development of the process, the methacrolein is obtained from propanal and formalin in the first stage. Such a procedure is in WO 2014/170223 described.

In U.S. 5,969,178 describes such a process for the oxidative conversion of isobutene or tert-butanol to methacrolein and the subsequent oxidative esterification to MMA. In this second stage, a liquid mixture of methacrolein and methanol with a reduced water content is reacted with molecular oxygen and a palladium catalyst, which is usually present on a support as a palladium-lead catalyst. From the crude product of the oxidative esterification, a mixture of methacrolein and methanol is then separated off below the top of the column in a first distillation stage, while low-boiling components are removed overhead. The bottom product containing MMA is then fed into a second distillation stage, in which an azeotrope of methanol and saturated hydrocarbons is removed overhead. The bottom, containing the crude MMA, is fed to a further work-up, while methanol is extracted from the fraction obtained overhead by means of a phase separator and a third distillation column isolated and returned to the reactor. It should be noted that the methanol can contain a relatively large amount of water due to the azeotrope formed and must therefore be dewatered.

As an alternative to this method, the U.S. 5,969,178 the work-up in only one column, in which case the feed is necessarily located above the bottom. Low-boiling constituents are removed from the reactor discharge at the top of this column. A mixture of crude MMA and water remains in the sump and is to be fed to further processing. A mixture of methacrolein and methanol, intended for recycling into the reactor, is finally removed from the column via a side stream, the exact position of which must first be determined and which can be adjusted by adding various boiling trays. U.S. 5,969,178 himself points out that such a process is difficult to carry out due to various azeotropes. In addition, methacrylic acid in particular, which is always present as a by-product, plays a major role. After this procedure, even if the U.S. 5,969,178 remains silent about this, the methacrylic acid would be separated off in such a way that it would remain in a phase to be disposed of and isolation would only be worthwhile to a limited extent. However, this reduces the overall yield of methacrylic products from this process.

In U.S. 7,012,039 a somewhat different processing of the reactor discharge from the oxidative esterification is disclosed. Here, in a first distillation stage, methacrolein is distilled off overhead via sieve trays and the aqueous, MMA-containing mixture is passed from the bottom into a phase separator. In this, the mixture is adjusted to a pH of about 2 by adding sulfuric acid. The sulfuric acid water is then separated from the organic or oil phase by means of centrifugation. In a further distillation, this organic phase is separated into high-boiling components and a phase containing MMA, which is drawn off overhead. The MMA-containing phase is then separated from low-boiling components in a third distillation. This is followed by a fourth distillation for final purification. The problem with this process is the sulfuric acid, which has to be added in large quantities and can have a corrosive effect on parts of the system. Accordingly, these parts, such as in particular the phase separator or also the second distillation column, must be made of suitable materials. Furthermore, she is also silent U.S. 7,012,039 about how to deal with the methacrylic acid that is produced at the same time or the remaining methanol in the product. However, it can be assumed that the former is also separated in the distillation stages, while the methanol can only be partially recovered and recycled with the methacrolein, while the remainder is probably lost in the third distillation stage.

WO 2014/170223 describes a similar procedure as U.S. 7,012,039 . The only difference is that in the actual reaction, the pH value is adjusted by adding a methanolic sodium hydroxide solution in a circuit. If the process is carried out without pH regulation, as described in the two publications mentioned, the reaction becomes "acidified". In a pH range below 7, this leads to Increased formation of the acetal of methacrolein, which would have to be separated off or split hydrolytically in a laborious process. In addition, the activity of the oxidation catalyst depends, among other things, on the pH value. At conditions below pH 7, the catalyst exhibits progressively lower activity as the pH of the reaction matrix decreases, which is undesirable. The pH regulation also serves, among other things, to protect the catalyst. Furthermore, the separation of the aqueous phase in the phase separation is easier due to the salt content. However, this also means that the methacrylic acid formed is present as a sodium salt and is later separated off with the aqueous phase and discarded. In the variant of adding sulfuric acid in the phase separation, the free acid is indeed recovered, but at the expense of the sodium (hydrogen) sulfate produced, which can lead to other problems during disposal.

Basically, and in summary of the prior art, various high-boiling components are obtained in the oxidative esterification of methacrolein or acrolein in the presence of methanol as alcohol, generally in the direct oxidative esterification of unsaturated aldehydes. These high boilers have higher boiling points than the methyl methacrylate (MMA) desired as the product and must therefore be separated from the MMA when the MMA is subsequently isolated in order to produce a suitable purity of the monomer of significantly more than 99% by weight. These by-products, in particular methacrylic acid, methyl methoxyisobutyrate, dimeric methacrolein (an aldehyde) and the corresponding ester of dimeric methacrolein, are formed in significant amounts and can have a clearly negative effect on the yield of the desired MMA. Since they have to be separated from the MMA that is produced, there is also a need to dispose of these substances, in the simplest case by incineration, thermal utilization with steam generation or biological degradation in a sewage treatment plant.

One of the by-products is methacrylic acid, which is formed in the DOE reaction in the presence of water, which is usually present in the reactor at a steady-state concentration of between 2 and 20% by weight when the reaction is carried out continuously. If the DOE reaction is carried out at constant pH, the methacrylic acid which forms is at least partially neutralized with alkaline or basic auxiliaries, in the simplest case with alkali compounds. Methoxyisobutyral, the Michael addition product of methanol to methacrolein, is also formed in the reaction. This methoxyisobutyral is at least partially converted under the conditions of the DOE in the presence of an oxygen-containing gas and, for example, methanol to form methyl methoxyisobutyrate (MIBS-M) as a side reaction. The reaction is alkaline catalyzed, so is inevitably formed by the addition of a base used to control the pH of the DOE reaction. It is generally observed that the formation of methoxyisobutyral and thus the consecutive formation of MIBSM are more pronounced with increasing pH. Depending on the catalyst used and depending on the chosen stationary pH of the DOE reaction in the reactor, between 0.1 and up to 5% of this by-product is formed.

In the state of the art, for example in German Auslegeschrift 1279015, Knörr et al. describes how alkoxypropionic acid alkyl esters can be cleaved thermally and catalytically, with unsaturated acrylic acid esters being obtained. Pure substances are used as educts and a pure β-elimination is described.

In the WO2016166525A1 Application describes Lucite the catalytic, base-induced cleavage of methyl methoxyisobutyrate in methanol and methyl methacrylate. Here, pure substances are used as educts in a protective gas atmosphere at 95 °C. However, the conversion is only at a low 37%.

Es resultiert in der Produktmatrix der DOE Reaktion eine Mischung, enthaltend Wasser, Methanol, MMA, freie Methacrylsäure neben alkalisch neutralisierter Methacrylsäure, beispielsweise Natrium Methacrylat, sowie den weiteren in Schema 2 beschriebenen Nebenprodukten.The following scheme 2 shows the reaction matrix (example with ethylene and syngas and formalin to form methacrolein; as described, access to methacrolein starting from isobutene or tert-butanol is also possible), in particular the formation of the target product MMA and the high-boiling by-products MAS, MIBSM, DIMAL and DIMAL esters are listed:

The result in the product matrix of the DOE reaction is a mixture containing water, methanol, MMA, free methacrylic acid in addition to alkaline-neutralized methacrylic acid, for example sodium methacrylate, and the other by-products described in Scheme 2.

• Möglichst hohe Ausbeute an MMA
• Umwandlung des Nebenprodukts Methacrylsäure zum Methylmethacrylat und Isolierung
• Umwandlung des Nebenprodukts Methoxyisobuttersäuremethylester (MIBSM) zu Methanol und MMA sowie Rückführung des entstehenden Methanols bzw. die Verwendung des MIBSM im Sinne einer Kreuzumesterung zur Umwandlung von MAS in MMA
• zumindest anteilige Umwandung von dimerem Methacrolein und dem entsprechenden DiMAL Ester in MMA und Methacrolein
• Möglichst hoher Grad des Recyclings von Nebenprodukten
• Möglichst saubere Entsorgungsströme bzw. Abgase

In summary, the following aspects of the methods according to the prior art are in need of improvement, especially in combination with one another:

• Highest possible yield of MMA
• Conversion of the by-product methacrylic acid to methyl methacrylate and isolation
• Conversion of the by-product methyl methoxyisobutyrate (MIBSM) to methanol and MMA and recycling of the resulting methanol or use of the MIBSM in the sense of a cross-transesterification to convert MAS into MMA
• at least partial conversion of dimeric methacrolein and the corresponding DiMAL ester into MMA and methacrolein
• Highest possible degree of recycling of by-products
• The cleanest possible disposal streams or waste gases

Task

In view of the prior art, it was therefore an object of the present invention to provide a technically improved process for the oxidative esterification of methacrolein which does not have the disadvantages of conventional processes and leads to higher yields than the prior art.

In particular, it was an object of the present invention to provide an improvement in the processing of the crude product of an oxidative esterification of methacrolein with methanol to give MMA and thus to improve the overall yield of such a process compared to the prior art.

In addition, it was an object to isolate as many by-products formed in the process as possible, in particular alkyl methoxyisobutyrate and methacrylic acid, to the greatest possible extent and to make them available for the production of alkyl methacrylates in a way that increases the yield. A further explicit object of the invention was to find a process which, if possible, allows several of these by-products to be converted into MMA in a single process step, and preferably builds on process steps in the processing of the crude product which are required in any case for the isolation of MMA . In particular, this relates to a solution to the problem that is based on columns, phase separators, extractors or general equipment that are already available for other reasons.

In particular, there was also the task of making available a method that can be operated with as little disposal effort as possible, in particular due to a reduced accumulation of organic components and acids in the waste stream.

Solution

The objects are achieved by a process for the preparation of alkyl methacrylates, in which methacrolein is prepared in a first reaction stage in a reactor I and this is oxidized in a second reaction stage in a reactor II with an alcohol, preferably with methanol, to form an alkyl methacrylate, preferably corresponding to MMA is esterified, the process being characterized according to the invention in that a. the reactor discharge from reactor II is separated into a fraction containing the majority of the alkyl methacrylate and a second fraction containing methacrylic acid and an alkyl alkoxyisobutyrate (AIBSM). In the preferred case where the alcohol is methanol, the AIBSM is methyl methoxyisobutyrate (MIBS-M).

Furthermore, the method according to the invention is characterized in that b. this second fraction is reacted in a reactor III in such a way that further alkyl methacrylate is formed from the AIBSM and the methacrylic acid. The chemical conversions are shown in Scheme 3, with the conversion being shown as an example using MAS and MIBS-M:

It can be shown very advantageously here that the cleavage of one by-product (MIBS-M) releases methanol, which is required in the sense of a cross-transesterification for the conversion of the second by-product, namely methacrylic acid MAS, in order to form MMA. The 3-methoxyisobutyric acid formed in traces is also first converted to 3-MIBS-M and then to MMA in this reaction.

All of these reactions take place synchronously, preferably in a single device or a single reactor, so that ultimately a large number of by-products are converted to the target product MMA.

The process for the synthesis of MMA, which has the two reaction stages listed above, can be used in particular in U.S. 5,969,178 , U.S. 7,012,039 and WO 2014/170223 be read. A possible flow diagram according to the invention is in 1 listed.

According to the invention, the first stage of the process for synthesizing the methacrolein can be freely selected. The process according to the invention can be used both for a first-stage synthesis based on tert-butanol or isobutylene and on the basis of propanal and formalin.

In the production of methacrolein based on propanal and formalin, there are in principle two possible process variants which, according to the invention, lead to a quality of the methacrolein which can be used in the DOE reaction. On the one hand, propanal and formalin can be reacted in a stirred or circulated reactor at temperatures of 20° C. to 120° C. at pressures of 1 bar to 10 bar. Reaction times of more than 10 minutes are required in order to achieve sufficient conversions. On the other hand, MAL can be produced from these educts, the desired high yields being achieved at medium pressure between 10 and 100 bar at higher temperatures between 120° C. and 250° C. with a reaction time of 2 seconds to 20 seconds.

The oxidative esterification is preferably carried out in the liquid phase at a pressure of 2 to 100 bar, preferably at a pressure in the range from 2 to 50 bar and a temperature in the range from 10 to 200° C. using a heterogeneous catalyst. The heterogeneous catalyst is usually supported, gold-containing nanoparticles with a particle size of less than 20 nm, preferably between 0.2 and 20 nm remaining propanal, and/or high boilers, such as dimeric methacrolein.

Particular preference is given to step a. the separation by means of at least one extraction and/or one distillation. It is also possible to use several distillation steps or extraction steps, as well as combinations of at least one distillation and at least one extraction.

It has proven to be very particularly preferred in the process according to the invention to carry out the process in such a way that the reactor discharge from reactor II is freed from methacrolein and partially from the alcohol in a first distillation column, with a stream containing alkyl methacrylate, water, an alkali metal methacrylate and /or methacrylic acid, MIBSM and alcohol. A strong acid is then added to this stream and, in an extraction, separated into a hydrophobic phase containing alkyl methacrylate, MAS and MIBSM and a hydrophilic phase containing water, the alcohol and remaining smaller amounts of main and by-products of the reaction.

Alternatively, and just as preferred, the process according to the invention is carried out in such a way that the reactor discharge from the reactor II is separated from methacrolein and in a first distillation column is partially de-alcoholized to obtain a stream containing alkyl methacrylate, water, an alkali methacrylate and/or methacrylic acid, MIBSM and alcohol. At this point in the process, the methacrylic acid can be present in the form of the free, organic acid or as an alkali metal salt or in the form of a mixture of free acid and alkali metal salt. Optionally, the acid present as a salt is converted into the free acid by acidification, for example by adding a Bronsted acid to the mixture.

This stream is then subjected to an extraction, resulting in an organic phase that mainly contains MMA, but also partially contains the organic by-products MAS and MIBSM. The organic phase resulting after extraction is separated in a second distillation into a low-boiling phase containing alkyl methacrylate and the alcohol and a high-boiling phase containing water, MIBSM and methacrylic acid.

Thus, in the process according to the invention, it is particularly advantageous to separate the valuable material MMA from the by-products of the DOE reaction, in particular in reactor II. In particular, a separation of the MMA from MAS, MIBSM, DIMAL and DIMAL ester and the isomers of HIBS can surprisingly be realized according to the invention.

According to the second aspect of the invention, this high-boiling phase described above is then subjected to a further reaction, with the by-products of the DOE being converted to the desired main product MMA. The additional MMA formed in reactor III increases the overall yield of the process significantly. The crude MMA product obtained in reactor III is advantageously fed to one or more work-up columns of the main process.

The reaction in reactor III is in turn preferably carried out at a temperature of at least 90.degree. C., particularly preferably at least 110.degree. C., very particularly preferably between 120.degree. C. and 170.degree. The reaction can thus be carried out purely thermally without the addition of a catalyst. The reaction is particularly preferably carried out thermally in the presence of a catalyst which can in particular be a Bronsted acid.

In a third alternative, the reaction can also take place in the presence of such a catalyst at temperatures below 130.degree.

It is particularly preferred if the Bronsted acid is a strong acid. According to the invention, a strong acid is understood as meaning an acid which is stronger than methacrylic acid. This means that this acid has a lower pKa value than methacrylic acid under normal conditions. A particularly preferred inorganic acid is sulfuric acid. The less preferred organic acids can be, for example, methanesulfonic acid or toluenesulfonic acid. An example of another suitable mineral acid is phosphoric acid. Sulfuric acid has proven to be a particularly suitable catalyst. In general, it is advantageous to use a catalyst acid in reactor III that is identical to the acid used to acidify and liberate the MAS from the alkali methacrylate.

A third function of the acid in this process is the decomposition of interfering acetals of methacrolein. Thus, in the best-case scenario, one is able to get by with just a single acid within the system network. A particularly suitable example of such a universally usable acid is sulfuric acid, which is concentrated or adjusted to different degrees depending on the application.

It has also proven preferable to combine the reactor effluent from reactor III with the alkyl methacrylate-containing fraction obtained after separating off the MIBSM and the methacrylic acid for further work-up. On the one hand, this second fraction can be the stream not contained in the MIBSM after the separation according to the invention in process step a). However, it is more favorable to pre-clean this stream in one or more steps before the two fractions are combined for further cleaning. The choice of pre-cleaning depends, for example, on the process chosen for the first process step in reactor I and the raw materials used therein. This preliminary purification of the actual crude alkyl methacrylate can be, for example, a high-boiling column, a low-boiling column or both distillations connected in series. Alternatively or in parallel, it is of course also possible to pre-clean the reactor discharge from reactor III before this fraction is combined with the other fraction mentioned above. This purification can also involve one or more extractions or distillations or combinations thereof. For this purpose, the acid used obtained from the purification can optionally be recycled back into the reactor III. A distillation column for separating off MAL is particularly preferably located directly downstream of reactor II. This can then be returned to reactor II or an upstream cleaning step.

According to the method, as a partial aspect of the present invention, the production of methacrolein in the reactor I described gives a good overview of the methods and processes for the production of methacrolein Ullmann's Encyclopedia of industrial chemistry, 2012, Wiley-VCH Verlag GmbH, Weinheim, DOI: 10.1002/14356007.a01_149.pub2 .

1. a. Herstellung von Methacrolein aus Propanal und Formalin in Gegenwart von Katalysatoren, bevorzugt homogenen Säuren, mineralisch oder organisch, und organischen Aminen unter erhöhtem, absolutem Druck von größer 2 bar. Beispiele sind unter anderem in EP 2 998 284 A1 oder in US 7,141,702 , JP 3069420 , JP 4173757 , EP 0 317 909 oder US 2,848,499 beschrieben. Auch geeignet ist das Verfahren gemäß DE 32113681 , wobei hohe Umsätze und Ausbeuten bei Verweilzeiten im Sekundenbereich erzielt werden. Bei letzter Publikation wird aber auch auf die Erzeugung von dimerem Methacrolein ("DIMAL") hingewiesen, was letztlich einen Verlust darstellt. Diese Verfahren verlaufen bei erhöhten Temperaturen und Drücken mit dem Vorteil einer niedrigen Verweilzeit der Reaktanden und somit vergleichsweise geringen Reaktorvolumina.
2. b. US 4,408,079 steht exemplarisch im Gegensatz zu den zuvor genannten Verfahren für Prozesse mit eher niedrigen Temperaturen und deutlich höheren Verweilzeiten, die somit höhere Reaktorvolumina benötigen. Diese Verfahren werden bei absoluten Drücken von über 2 bar durchgeführt. Gleichzeitig werden aber deutlich höhere Katalysatormengen - teilweise von sogar 50 mol% - benötigt, wobei aber die Katalysatorlösungen auch recycliert werden können. Üblicherweise werden bei diesen Verfahrensvarianten gerührte oder umgepumpte Rührkessel oder Kaskaden dieser Kessel verwendet. Kennzeichnend bei dieser Verfahrensvariante sind eher höhere DIMAL-Gehalte als Nebenprodukt, was auf längere Verweilzeiten und somit verstärkt vorkommende Diels-Alder-Reaktion von Methacrolein zurückzuführen ist.
3. c. Die dritte Verfahrensvariante zur Herstellung von Methacrolein ist dadurch gekennzeichnet, dass Isobuten oder tert-Butanol in der Gasphase an einem heterogenen Kontakt mit Wasserdampf und sauerstoffhaltigen Gasen bei Temperaturen von über 300°C umgesetzt und anschließend isoliert wird. Eine Vielzahl von Untervarianten und einsetzbarer Katalysatorsysteme sowie Isolierungsoptionen sind im einschlägigen Stand der Technik beschrieben. Eine gute Übersicht hierzu gibt folgende Referenz: Trends and Future of Monomer-MMA Technologies, K. Nagai & T. Ui, Sumitomo Chemical Co., Ltd.,Basic Chemicals Research Laboratory, 2005, http://ww.sumitomo chem.co.jp/english/rd/report/theses/docs/20040200_30a.pdf . werden Diese Verfahren mit C4 -Rohstoffen werden vor allem im asiatischen Raum großtechnisch durchgeführt.

We were able to show that several process variants are in principle suitable for producing methacrolein, which can look like this:

1. a. Production of methacrolein from propanal and formalin in the presence of catalysts, preferably homogeneous acids, mineral or organic, and organic amines under increased absolute pressure of more than 2 bar. Examples include in EP 2 998 284 A1 or in U.S. 7,141,702 , JP 3069420 , JP 4173757 , EP 0 317 909 or U.S. 2,848,499 described. The method according to is also suitable UK 32113681 , with high conversions and yields being achieved with residence times in the seconds range. In the last publication, however, reference is also made to the production of dimeric methacrolein ("DIMAL"), which ultimately represents a loss. These processes run at elevated temperatures and pressures with the advantage of a short residence time for the reactants and thus comparatively small reactor volumes.
2. b. U.S. 4,408,079 stands in contrast to the previously mentioned processes for processes with rather low temperatures and significantly longer residence times, which therefore require higher reactor volumes. These processes are carried out at absolute pressures in excess of 2 bar. At the same time, however, significantly higher amounts of catalyst—sometimes even 50 mol%—are required, although the catalyst solutions can also be recycled. Stirred or circulated stirred tanks or cascades of these tanks are usually used in these process variants. This process variant is characterized by higher DIMAL contents as a by-product, which can be attributed to longer residence times and thus the increased occurrence of the Diels-Alder reaction of methacrolein.
3. c. The third process variant for the production of methacrolein is characterized in that isobutene or tert-butanol is reacted in the gas phase in heterogeneous contact with steam and oxygen-containing gases at temperatures above 300° C. and is then isolated. A large number of sub-variants and usable catalyst systems as well as isolation options are described in the relevant prior art. The following reference provides a good overview of this: Trends and Future of Monomer-MMA Technologies, K. Nagai & T. Ui, Sumitomo Chemical Co., Ltd., Basic Chemicals Research Laboratory, 2005, http://ww.sumitomo chem.co.jp/english/rd/report /theses/docs/20040200_30a.pdf . These processes with C4 raw materials are mainly carried out on an industrial scale in Asia.

It is preferred that the first reaction stage in reactor I is a reaction of propanal with formalin. In this case, it is then further preferred that the reactor discharge from reactor II is freed from methacrolein and partially from the alcohol in a first distillation column, a stream comprising alkyl methacrylate, water, an alkali metal methacrylate and/or methacrylic acid, MIBSM and alcohol being obtained becomes. This stream is then subsequently separated in a second distillation into a light phase containing alkyl methacrylate and the alcohol and a heavy phase containing water, MIBSM, methacrylic acid, dimeric methacrolein and optionally an alkyl ester of dimeric methacrolein.

The dimeric methacrolein in reactor III in this process variant can then be split into methacrolein. It is then also possible to split the optionally present alkyl ester of the dimeric methacrolein into methacrolein and the alkyl methacrylate corresponding to the alcohol used. The methacrolein thus obtained can then be separated from the alkyl methacrylate in a later distillation stage and returned to reactor II.

As an alternative to the process variant described, in which the first reaction stage in reactor I is a reaction of propanal with formalin, this first reaction stage can also be an oxidation of tert-butanol and/or isobutene.

In such a variant, it is then particularly preferred to free the reactor discharge from reactor II in a first distillation column from methacrolein and partly from the alcohol. This gives a stream which contains alkyl methacrylate, water, an alkali metal methacrylate and/or methacrylic acid, MIBSM and alcohol. This stream is usually first treated with acid, for example mineral acids such as sulfuric acid, neutralizing most of the alkali metal methacrylate and thus forming the free methacrylic acid.

This stream is then subsequently in a second distillation and / or extraction in a light or hydrophobic phase containing alkyl methacrylate and the by-products of the reaction, namely the majority of the methacrylic acid formed, MIBSM, and DIMAL, DIMAL ester and smaller amounts of water and methanol, and a heavy, hydrophilic phase separated. This second, predominantly aqueous phase naturally contains few organic products and consists mainly of water and methanol and contains alkali or alkaline earth salts from the neutralization.

As a partial aspect of the present invention, methacrolein is reacted in the presence of an oxygen-containing gas at moderate temperatures between 20 and 150° C. at moderate pressures between 1 and 20 bar in the presence of a heterogeneous spherical catalyst containing noble metals. A variety of catalysts can be used for this oxidative esterification of MAL with methanol to MMA:
The first known application of the direct oxidative esterification of MAL with methanol to MMA was carried out by Asahi with a Pd-Pb catalyst, which is located on an oxidic support. In the U.S. 6,040,472 these catalysts are described, which, however, lead to MMA with only insufficient activities and selectivities in comparison. In this case, it is a matter of Pd-Pb-containing catalysts with a shell structure. The selectivity to MMA is given as up to 91% and the space-time yield as up to 5.3 mol. Here, too, the lead doping is decisive for the formation of the active oxidation species, but produces the disadvantages described above due to the gradual loss of lead ions. By-products of these catalysts, as with all other systems, are methacrylic acid and other by-products.

In EP 1 393 800 gold-containing catalysts are described, with the catalytic gold particles described as the active oxidation species having to have an average diameter of less than 6 nm in particular. Said gold particles are distributed on a silicon oxide or a TiO ₂ /SiO ₂ carrier. In addition to gold, such catalysts also contain other metals in oxidic form as additional active components. this one A synergistic and activity- and selectivity-increasing effect is ascribed to doping components.

It is produced by applying the gold salt and other metal salts to an oxidic carrier.

Haruta et al in J. Catal. 1993, Vol. 144, pp. 175-192 that gold nanoparticles applied to transition metal oxide carriers such as TiOz, Fe ₂ O ₃ or Co ₃ O ₄ represent active oxidation catalysts. An interaction between gold and transition metal plays a crucial role in catalyst activity.

In EP 2 177 267 and EP 2 210 664 describes nickel-containing catalysts with a shell structure. Selectivity to MMA is up to 97% with these catalysts. The space-time yield is described as 9.7 mol MMA/(kg h) with a gold content in the catalyst of about 1% by weight. According to examples, a NiO _x /Au catalyst shows significantly better activities and selectivities for MMA, while other combinations, such as Au with CuO or Co ₃ O ₄ , are much less active and selective. Basically, this is a further development of the catalysts described above, with the inhomogeneous distribution of the gold-Ni-oxide composite particle and an inactive catalyst shell characterizing these new catalysts. These also refer to the formation of methacrylic acid as a by-product.

In EP 2 210 664 a catalyst is disclosed which, in the form of a so-called egg-shell structure, has nickel oxide and gold nanoparticles on a support made of SiOz, Al ₂ O ₃ and a basic element, in particular an alkali metal or alkaline earth metal, on the outside. The nickel oxide is enriched on the surface, but is also present in lower concentrations in the deeper layers of the catalyst particle.

In WO 2017/084969 A1 describes catalyst systems based on several mixed oxides as carriers, which also have nanoparticulate gold in addition to cobalt oxide as an active component. The distribution of the catalytically active components, namely gold and cobalt, are distributed anisotropically over the cross section of the grain. Other newer catalysts for said reaction are in U.S. 9,676,699 described. Similar carrier systems based on silica, alumina and alkaline earth oxide mixtures, which contain palladium in addition to bismuth with various third-party dopings, are described here. Here, too, reference is made to the connection between the water concentration and the methacrylic acid that forms as a by-product.

As different as these catalyst systems are with regard to the support materials used, their preparation and ultimately also their performance in terms of conversion and selectivity are just as different. Nonetheless, all of these catalyst systems result in similar by-product characteristics. What all catalysts have in common is that, in addition to the desired alkyl methacrylate, more or less large amounts of methacrylic acid, Alkoxyisobutyric acid ester (AIBSM), and form dimers of the methacrolein used and alkyl esters of these dimers. Other by-products such as hydroxyisobutyric acid and its corresponding esters are also formed. These by-products are relevant above all because they are high-boiling compared to the desired alkyl methacrylate and accumulate in high-boiling fractions and ultimately collect during work-up compared to MMA.

List of references to Figure 1

1. (1) Reaktor I zur MAL-Synthese
2. (2) Destillationskolonne
3. (3) Reaktor II zur DOE Reaktion
4. (4) MAL Abtrennung
5. (5) Zwischenkolonne und/oder Extraktion
6. (6) Kolonne zur Methanol-Abtrennung
7. (7) Kolonne zur MMA-Aufreinigung - Schwersieder
8. (8) 2. Kolonne zur MMA-Aufreinigung - Leichtsieder
9. (9) 3. Kolonne zur MMA-Aufreinigung - Reinkolonne
10. (10) Gereinigtes MMA
11. (11) Optionale Kolonne zur Reduzierung der MMA Menge aus Sumpfstrom von (7)
12. (12) Zugabe Säure und MeOH für (13)
13. (13) Reaktor III zur Spaltung von MIBSM in MMA und DIMAL-Ester in MAL und MMA sowie Veresterung von MAS zu MMA
14. (14) Optionale Kolonne zur Trennung der Wertstoffe von Schwersiedern & Schwefelsäure
15. (15) Abwasser
16. (16) Rückführung für Methacrolein und Methanol

figure 1 is an example of a possible flow diagram of the MMA process according to the invention, including the reactor for obtaining MMA from MIBSM and MAS.

1. (1) Reactor I for MAL synthesis
2. (2) Distillation Column
3. (3) Reactor II for DOE reaction
4. (4) MAL separation
5. (5) Intermediate column and/or extraction
6. (6) Methanol separation column
7. (7) MMA purification column - high boilers
8. (8) 2nd column for MMA purification - low boilers
9. (9) 3rd column for MMA purification - clean column
10. (10) Purified MMA
11. (11) Optional column to reduce the amount of MMA from the bottom stream of (7)
12. (12) Add acid and MeOH for (13)
13. (13) Reactor III for cleavage of MIBSM into MMA and DIMAL ester into MAL and MMA and esterification of MAS into MMA
14. (14) Optional column for separating valuable materials from high boilers and sulfuric acid
15. (15) waste water
16. (16) Recycle for methacrolein and methanol

Experimental part: Example 1 Carrying out the reaction at normal pressure as a feed batch with fresh starting materials, MAS and MIBSM.

The reaction is carried out in a three-necked glass flask fitted with a column. The three-necked flask is equipped with a KPG stirrer and a 1 m high column with a free diameter of 40 mm, which is heated via an oil bath. The column is filled with Raschig rings, and a reflux divider is fitted at the top of the column section in order to and to be able to control acceptance. 2 moles of MIBSM and 2 moles of methacrylic acid are placed in a 1 l three-necked flask, as well as 0.2 mole of water.

200 ppm of phenothiazine and 50 ppm of Tempol are added to this mixture as stabilizers or to inhibit free-radical polymerization of (meth)acrylic starting materials and products under the reaction conditions. The reaction mixture is heated to 150° C. using an oil bath, after 10 minutes this temperature is reached with the preheated oil bath, the column is set to full reflux, so that initially no distillate is obtained. After the internal temperature of 150° C. has been reached, a mixture of MIBSM, MAS and MeOH, water and sulfuric acid is fed continuously into the reaction mixture at a metering rate of 150 g/h via the submerged capillary. The feed mixture is dosed via an HPLC pump, a second HPLC pump discharges the resulting reaction bottoms via a capillary after the reaction has been run in stationary.

Educts as well as catalyst, alcohol and water are premixed separately and introduced into the reaction via a capillary that is routed under the stirrer. Composition of the feed mixture: <b>Table 1: Feed mixture for the cleavage of MIBSM to MMA with parallel esterification of MAS to MMA.</b> chemical wt% M [g/mol] Feed [g/h] Feed [mol/h] Mol% [Base = MIBSM) MIBSM 44 118 66 0.56 100 M.A.S 32 86 48 0.56 100 Water 3.3 18 4.95 0.28 49 sulfuric acid 3.7 98 5.55 0.06 10 methanol 17 32 25.5 0.80 142

Thus, the molar ratio of the C4 by-products MAS and MIBSM corresponds to 1:1. The water content is 49 mol% based on MIBSM and 142 mol% MeOH based on MIBSM.

The oil bath is heated to 160°C with the start of feed dosing, the internal temperature in the reactor gradually rises to around 150°C and a mixture consisting of the azeotropic compositions of the binary azeotropes MeOH and MMA or MMA and water collects at the top of the column. As soon as the top of the column has reached a stable temperature of 69° C., a reflux ratio of 0.8 is set and the distillate is taken off.

The reaction is initially operated continuously for 6 hours, with the amount of distillate being quantified and analyzed every hour. The system is operated in such a way that on average about 90% of the reactant mass fed in is drawn off as distillate at the top per hour, while the reaction bottoms are also continuously discharged. An average of about 10% of the feed stream fed in is removed by means of an HPLC pump. On average, the fill level in the flask remains the same and the reaction at this stage is to be considered stationary in terms of volume or mass. At the top of the column downstream of the condenser, which is supplied with tap water a cooling water temperature of about 18°C, a cold trap is installed to capture volatile components; the cold trap is operated with a mixture of acetone/dry ice at almost minus 60°C, the cold trap is filled with THF to absorb volatile components and to clarify and determine them qualitatively.

The bottom turns yellowish at first, then light orange within 6 hours, and there is hardly any noticeable increase in viscosity.

The column top product obtained stationary as distillate weighs 134.9 g/h and, according to GC chromatography, is composed as follows: <b>Table 2: Distillate product of the reaction.</b> chemical wt% M [g/mol] Distillate [g/h] Distillate [mol/h] mma 79.1 100 106.7 1.06 Water 3.3 18 4.7 0.26 methanol 17.6 32 23.7 0.74 distillate total 100 # 134.9 2.06

The stationary bottom product from the discharge weighs 15.1 g/h and is composed as follows: <b>Table 3: Bottom product of the reaction.</b> chemical wt% M [g/mol] sump [g/h] sump [mmol/h] MIBSM 17.5 118 2.64 22:4 M.A.S 21:9 86 3.30 38.4 methanol 0.1 32 0.01 0.3 sulfuric acid 36.8 98 5.55 56.6 Water 1.7 18 0.25 13.9 high boilers 22.2 # 3.35 # swamp total 100 # 15.1 #

This corresponds to a calculated yield of MMA based on MIBSM of 96% based on the amounts of starting material fed in, based on the methacrylic acid esterified to MMA of 95% and a methanol recovery rate of 93%.

In addition to the THF solvent used as the absorbent, small amounts of dimethyl ether are detected in the cold trap by means of gas chromatography (boiling point -24° C.).

The experiment shows that under the selected conditions from mixtures containing MAS and MIBSM in the presence of stoichiometric amounts of sulfuric acid and in the presence of MeOH and water, raw MMA can be produced highly efficiently and at high conversion rates (regarding the starting materials).

Example 2 to 9:

Preparation of methacrolein from propanal and formalin: Methacrolein was prepared according to EP 2 998 284 manufactured and isolated.

A formalin solution with a formalin content of 37% by weight or 55% by weight, depending on the example, and propanal are mixed using a static mixer (hereinafter referred to as aldehyde solution) and then heated to the desired temperature in an oil-heated heat exchanger (see Table 1). . The exact water content of the formalin, depending on the example, is of no further importance, as this is fully included in the water content of the fresh feed according to Table 1. A recycle stream from the bottom of the product column, which is connected to the tubular reactor, is mixed with acetic acid and dimethylamine (as a 40% strength solution in water) and likewise preheated to the desired temperature. The preheated aldehyde solution and the preheated catalyst solution are mixed in another static mixer. This educt mixture is then fed to a tubular reactor that is heated by means of oil. The reaction is usually carried out at pressures of about 35 to 40 bar.

The product mixture at the outlet of the tubular reactor is expanded via a valve and reaches the product column for distillation. After condensation and phase separation, a two-phase mixture of methacrolein and an aqueous phase is obtained at the top of this column. The aqueous phase is returned to the column. The organic phase enters the product receiver. At the bottom of the column, a partial stream is returned to the reaction as recycle. Another partial flow is discharged as an aqueous product into another product receiver. In Examples 1 to 4, a methacrolein quality with a DIMAL content of less than 0.2% by weight is obtained. The water content is about 56% by weight and the dimethylamine content, based on the water in the feed, is about 2.7% by weight. The temperature in the reactor is between 122°C as the inlet temperature and 153°C as the outlet temperature. A significant temperature peak does not occur.

Examples 5 to 7 show that the parameters of the reaction process have a significant influence on conversion and DIMAL content, since a content of dimeric MAL below 0.4% by weight could be achieved here, but not below 0.2% by weight. The difference to Examples 1 to 4 is the higher maximum temperature and the higher outlet temperature, in addition to a partially higher inlet temperature.

Examples 8 and 9 show embodiments which produce a methacrolein quality with a content of dimeric MAL below 0.5% by weight. Here the inlet temperatures and especially the maximum temperatures were even higher. In particular, the maximum temperatures were above the preferred maximum temperatures of 165°C or even 170°C. <b>Table 4: Preparation of MAL from propionaldehyde and formalin.</b> Table: Production of methacrolein from propanal (PA) and formalin (FO) PA:FO DMA:PA ACOH:DMA recycle DMA:PA _H2O DMA/H2O VWZ T _OIL T _{a max} T _off Sales PA selectivity TIMES c DIMAL fresh feed reactor inlet mol/mol mol% mol/mol % mol% % % sec °C °C °C % % % DE3213681A1 Ex1 1 3.7 1.08 - - 50 1.8 6.9 161 184 - 99.5 98.1 0.49 DE3213681A1 Ex2 1 3.6 1.14 - - 40 2.5 6 162 205 - > 99.4 97.2 <1 example 1 0.99 2.50 1.09 70.5 7.8 55.6 2.74 9.30 139.5 122.5 152.6 152.2 99.37 98.75 0.18 example 2 0.99 2.51 1.09 71.0 7.8 56.1 2.74 9:26 139.1 122.5 152.3 152.0 99.30 98.85 0.18 Example 3 0.98 2.61 1.09 71.2 8.2 54.9 2.82 9:41 139.9 122.1 152.3 152.2 99.35 98.67 0.18 example 4 0.96 2.51 1.09 70.1 7.7 56.5 2.71 9:21 139.1 122.8 153.0 153.0 99.46 98.33 0.18 Example 5 0.99 2.51 1.09 70.5 7.8 55.7 2.75 9:26 143.9 129.9 160.2 155.5 99.75 98.19 0.34 Example 6 0.99 2.51 1.09 70.4 7.8 55.6 2.75 9.30 144.2 127.3 157.7 154.7 99.65 98.47 0.27 Example 7 0.98 2.50 1.09 70.5 7.7 56.0 2.72 9:22 139.0 122.5 156.3 154.9 99.57 98.62 0.22 example 8 0.99 2.51 1.09 70.5 7.8 55.6 2.74 9:26 159.8 142.1 173.0 169.1 99.67 98.03 0.49 example 9 0.99 2.52 1.08 70.4 7.8 55.6 2.76 9:26 146.4 133.8 165.4 159.7 99.77 98.34 0.45

After the reaction, the methacrolein prepared as described above is expanded (optionally partially evaporated in a flash box) and fed into a distillation column. After condensation, a two-phase mixture is obtained at the top of the distillation column (depending on the temperature, a more or less large water phase separates), the upper phase containing methacrolein in a quality of >97% with a water content of 1-3 wt%. The formalin content in the methacrolein is <2000 ppm, the methanol content is between 0.1 and 1.0 wt%, depending on the methanol content of the formalin used. According to the above examples, the methacrolein contains a DiMAL content of 0.18 wt% to <1 wt%. This quality is used in the following experiments for the direct oxidative esterification with methanol.

Example 10 Carrying out the direct oxidative esterification in the liquid phase

1. a) Als Katalysator wird ein Gold-Cobaltoxid Katalysator ( WO2017084969 A1 ) eingesetzt und ein Umsatz von 76% MAL bei einer Selektivität von 94,1% MMA erhalten wird. Die Selektivität zu MAS beträgt 3,1% und die Selektivität zu MIBSM 1.2%
2. b) Als Katalysator wird ein Gold-Nickeloxid Katalysator ( US8450235 ) eingesetzt und ein Umsatz von 75% MAL bei einer Selektivität von 94,4% MMA erhalten wird. Die Selektivität zu MAS beträgt 2,5% und die Selektivität zu MIBSM 1.2%
3. c) Als Katalysator wird ein Palladium-Blei Katalysator ( US5969178 ) eingesetzt, wobei der pH-Wert der Reaktion auf 6.3 eingestellt wird ist und ein Umsatz von 60% MAL bei einer Selektivität von 89% MMA erhalten wird. Die Selektivität zu MAS beträgt 7% und die Selektivität zu MIBSM liegt unter 0,1%.
4. d) Als Katalysator wird ein Palladium-Bismuth-Tellurium Katalysator ( US20160168072 ) eingesetzt und die Bedingungen und Stöchiometrien werden gemäß Beispiel 2 und 3 wie in US20160168072 beschrieben, eingestellt. Man erhält einen Umsatz von 89% bei einer Selektivität von 92% MMA. Die Selektivität zu MAS liegt unterhalb von 0.2% und die Selektivität zu MIBSM liegt bei 1.2%.

A 20 l reactor with gassing stirrer is filled with a reaction mixture of 36 percent by weight methacrolein in methanol at a slurry density of 8 percent by weight catalyst. The reaction mixture is brought to 5 bar at 80° C. with stirring and air is metered in such that the oxygen concentration in the residual gas after the condensers is 4.0 percent by volume. The pH is adjusted to 7 by continuous addition of 4% by weight NaOH in methanol solution. The reaction mixture is continuously discharged from the reactor in such a way that the throughput over the catalyst is 11 mol MAL/kg catalyst/hour. The running time is 1000 hours each.

1. a) A gold-cobalt oxide catalyst ( WO2017084969 A1 ) used and a conversion of 76% MAL at a selectivity of 94.1% MMA is obtained. The selectivity to MAS is 3.1% and the selectivity to MIBSM is 1.2%
2. b) A gold-nickel oxide catalyst ( US8450235 ) used and a conversion of 75% MAL at a selectivity of 94.4% MMA is obtained. The selectivity to MAS is 2.5% and the selectivity to MIBSM is 1.2%
3. c) A palladium-lead catalyst ( US5969178 ) used, the pH of the reaction is adjusted to 6.3 and a conversion of 60% MAL at a selectivity of 89% MMA is obtained. The selectivity to MAS is 7% and the selectivity to MIBSM is less than 0.1%.
4. d) A palladium-bismuth-tellurium catalyst ( US20160168072 ) used and the conditions and stoichiometries are according to Example 2 and 3 as in US20160168072 described, adjusted. A conversion of 89% with a selectivity of 92% MMA is obtained. The selectivity to MAS is below 0.2% and the selectivity to MIBSM is 1.2%.

Example 11 Cleavage of MIBSM to form MMA and methanol with simultaneous esterification of MAS to form MMA using reaction mixtures from Example 3 after separation of methacrolein and methyl methacrylate according to the invention. Sulfuric acid as a catalyst

The reaction mixture obtained from Example 3a was taken after work-up:
The work-up is described as an example, with the respective compositions being listed in Table 1.

The discharge from reactor II (1000 g/hr) was passed to stage 11 of 22 of the MAL recovery column. The temperature in the sump was 70° C. at a pressure of 930 mbar. The bottom stream was acidified to pH 2 with sulfuric acid, separated in a decanter and the organics were fed into the bottom of the extraction column, with the aqueous phase being introduced into the top of the 30-stage extraction column. The bottom temperature of the extraction column was 43.9° C. at a pressure of 1013 mbar. The top stream of the extraction column was introduced to stage 6 of 10 of the high-boiling column. The bottom temperature was 85.4° C. at a pressure of 235 mbar. <b>Table 5: Composition of the reaction mixture in the various work-up steps.</b> component reactor discharge Swamp MAL recovery head extraction swamp heavy boiler methanol 47.3% 28.9% 1.8% 22 ppm Water 5.6% 10.9% 6.0% 0.3% JUST 10.3% 64 ppm 1300 ppm 12 ppm mma 32.6% 55.1% 83.4% 11.4% MIBSM 0.4% 1.2% 1.4% 32.2% M.A.S 0.9% 0.6% 2.8% 24.6% DIMAL ester 0.1 0.3% 0.7% 12.8% minor components 2.8% 2.1% 3.8% 19.0%

The bottom of the high boiler column was collected and used for splitting MIBSM into MMA and MeOH and DIMAL ester into MAL and MMA with simultaneous esterification of MAS with MeOH to form MMA.

• 300 g Feed (1 Eq., 0,73 mol 3-MibsM; 1,17 Eq., 0.85 mol MAS; 0,34 mol MMA, 0,23 mol DIMAL-Ester),
• 2,84 g (0,04 Eq., 0,029 mol) H₂SO₄ und
• 13,42 g H2O (1,02 Eq., 0,75 mol)
• wurden in dem Dreihalskolben vorgelegt, das Ölbad (Soll Temperatur = 165 °C) geführt wurde.

A 500 mL three-necked flask was fitted with a column and a glass thermometer. At the top of the column, 50 g of MeOH with phenothiazine (about 500 ppm) were placed in a dropping funnel in order to prevent polymerization in the column by continuous addition. The thermocouple was placed in the oil bath (T(OI)=165°C).

• 300 g feed (1 eq., 0.73 mol 3-MibsM; 1.17 eq., 0.85 mol MAS; 0.34 mol MMA, 0.23 mol DIMAL ester),
• 2.84 g (0.04 eq., 0.029 mol) H ₂ SO ₄ and
• 13.42 g H2O (1.02 eq., 0.75 mol)
• were placed in the three-necked flask, the oil bath (set temperature = 165 ° C) was performed.

The mixture was heated to 165° C. (oil bath target) for 3 h, a bottom temperature of 151° C. being reached. The distillate removal took place continuously and was analyzed by HPLC. Over the course of the 3 hour reaction, the methanol stabilizer solution (6.54 g/h, 0.20 mol) was added

Table No.2 below shows the amounts of sulfuric acid, water, methanol and feed sample used. The composition of the feed sample for 3-MibsM, MAS and MMA together with the molar masses is also listed. <b>Table 6: Amount of substances used and their molar masses.</b> Feed [wt%] H ₂ SO ₄ [% by weight] H ₂ O [wt%] MeOH [wt%] Stability [% by weight] total [g] Molar mass 3-MibsM 32.31 132:16 M.A.S 24.61 86.09 mma 11:41 100.12 DIMAL ester 12.8 170.21 H2SO4 98.00 98.08 H2O 2.00 100.00 18.02 MeOH 100.00 32.04 S47 S71 masses [g] 300.51 2.84 13:42 19.63 0.26 362.57

Table No.7 shows the recovery of the masses used for the starting materials listed above. <b>Table 7: Mass balance closure</b> Amount used [g] Distillate [g] swamp [g] dest+sump [g] difference [g] accounting 362.57 232.81 125.78 358.59 3.98 98.90%

The mass balance is at a recovery of 98.90%. Table No.8 shows the molar distribution of the components in the distillate and bottom. <b>Table 8: Distribution of the components in the distillate and bottom</b> comp. Template [mol] dist. [mol] sump [mol] Delta Mol turnover/sel. 3-MIBSM 0.735 0.000 0.029 0.705 96.00% Sales to 3-MIBSM M.A.S 0.859 0.001 0.108 0.750 87.30% Sales to MAS mma 0.342 1,620 0.166 1,443 99.16% Selectivity/(MAS+3-MIBSM) JUST 0 0.019 0 0.019 8.26% Conversion of DIMAL esters to MAL and MMA MeOH( ^∗ ) 0.613 0.23 - - H2O 0.753 - -

The trial was successful, with high conversion of 3-MIBSM and MAS to MMA.

There was a conversion of 96.0% 3-MIBSM to MMA and 87.3% conversion of MAS to MMA. The total selectivity to MMA for MAS and 3-MIBSM was 99.16%. The thermal cleavage conversion of DIMAL ester to MAL and MMA was 8.26%.

Example 12 Cleavage of MIBSM to form MMA and methanol with simultaneous esterification of MAS to form MMA using reaction mixtures from Example 3 after separation of methacrolein and methyl methacrylate according to the invention. phosphoric acid as a catalyst

The reaction was carried out analogously to Example 11 and phosphoric acid was used instead of sulfuric acid.

The trial was successful, with high conversion of 3-MIBSM and MAS to MMA. There was a conversion of 94.2% 3-MIBSM to MMA and 86.8% conversion of MAS to MMA. The total selectivity to MMA for MAS and 3-MIBSM was 99.0%. The thermal cleavage conversion of DIMAL ester into MAL and MMA was 8.25%.

Example 13 Cleavage of MIBSM to form MMA and methanol with simultaneous esterification of MAS to form MMA using reaction mixtures from Example 3 after separation of methacrolein and methyl methacrylate according to the invention. Methanesulfonic acid as a catalyst

The reaction was carried out analogously to Example 11 and methanesulfonic acid was used instead of sulfuric acid.

The trial was successful, with high conversion of 3-MIBSM and MAS to MMA. There was a conversion of 92.6% 3-MIBSM to MMA and 84.7% conversion of MAS to MMA. The total selectivity to MMA for MAS and 3-MIBSM was 98.8%. The thermal cleavage conversion of DIMAL ester into MAL and MMA was 8.20%.

Example 14 Cleavage of MIBSM to form MMA and methanol with simultaneous esterification of MAS to form MMA using reaction mixtures from Example 3 after separation of methacrolein and methyl methacrylate according to the invention. temperature of 120 °C

The reaction was carried out analogously to example 11 at a temperature of 120.degree.

There was a high conversion of 3-MIBSM and MAS to MMA.

There was a conversion of 60.4% 3-MIBSM to MMA and 87.3% conversion of MAS to MMA. The thermal cleavage conversion of DIMAL ester into MAL and MMA was 8.20%.

Example 15 Cleavage of MIBSM to form MMA and methanol with simultaneous esterification of MAS to form MMA using reaction mixtures from Example 3 after separation of methacrolein and methyl methacrylate according to the invention. temperature of 90 °C

The reaction was carried out analogously to example 11 at a temperature of 90.degree.

There was no high conversion of 3-MIBSM to MMA; but from MAS to MMA.

There was a conversion of less than 1% 3-MIBSM to MMA and 88.0% conversion of MAS to MMA. The thermal cleavage of DIMAL ester into MAL and MMA did not proceed.

Example 16 Cleavage of MIBSM to form MMA and methanol with simultaneous esterification of MAS to form MMA using reaction mixtures from Example 3 after separation of methacrolein and methyl methacrylate according to the invention. Increased amount of sulfuric acid

The reaction was carried out analogously to Example 11 with an increased amount of sulfuric acid of 40 mol%.

The trial was successful, with high conversion of 3-MIBSM and MAS to MMA. There was a conversion of 95.9% 3-MIBSM to MMA and 87.4% conversion of MAS to MMA. The total selectivity to MMA for MAS and 3-MIBSM was 99.14%. The thermal cleavage conversion of DIMAL ester into MAL and MMA was 8.16%.

comparative example. 1: Cleavage of MIBSM to MMA and methanol with simultaneous esterification of MAS to MMA with reaction mixtures from Example 3 after separation of methacrolein and methyl methacrylate according to the invention. temperature of 23 °C

The reaction was carried out analogously to example 11 at a temperature of 23.degree.

There were no high conversions of 3-MIBSM to MMA and MAS to MMA.

There was a conversion of less than 1% 3-MIBSM to MMA and 20% conversion of MAS to MMA. The thermal cleavage of DIMAL ester into MAL and MMA did not proceed.

comparative example. 2: Cleavage of MIBSM to MMA and methanol with simultaneous esterification of MAS to MMA with reaction mixtures from example 3 after separation of methacrolein and methyl methacrylate according to the invention. No addition of sulfuric acid as a catalyst

The reaction was carried out analogously to Example 11 without adding sulfuric acid as a catalyst.

There was high conversion of 3-MIBSM to MMA but no conversion of MAS to MMA. There was 94% conversion of 3-MIBSM to MMA and no conversion of MAS to MMA. The thermal cleavage of DIMAL ester into MAL and MMA was 8.0%.

Example 17 Continuous cleavage of MIBSM to form MMA and methanol with simultaneous esterification of MAS to form MMA using reaction mixtures from Example 3 after separation of methacrolein and methyl methacrylate according to the invention.

The reaction was carried out analogously to Example 11. In addition, after the bottom temperature of 151° C. had been reached, a continuous feed of feed mixture at a rate of 57 g/hr was started. The experiment ran for 6 hours.

The trial was successful, with high conversion of 3-MIBSM and MAS to MMA. There was a conversion of 95.5% 3-MIBSM to MMA and 87.5% conversion of MAS to MMA. The total selectivity to MMA for MAS and 3-MIBSM was 98.7%. The thermal cleavage conversion of DIMAL ester into MAL and MMA was 8.05%.

Claims (18)

1. Process for preparing alkyl methacrylates, in which methacrolein is prepared in a first reaction stage in a reactor I and this is oxidatively esterified with an alcohol in a second reaction stage in a reactor II to give an alkyl methacrylate, characterized in that

   a. the reactor output from reactor II is separated into a fraction containing the predominant portion of the alkyl methacrylate and a second fraction containing methacrylic acid and an alkyl alkoxyisobutyrate, and in that

   b. this second fraction is converted in a reactor III in such a way that further alkyl methacrylate is formed from the alkyl alkoxyisobutyrate and the methacrylic acid.
2. Process according to Claim 1, characterized in that the separation in step a is effected by means of at least an extraction and/or a distillation.
3. Process according to Claim 1 or 2, characterized in that the reaction in step b. is conducted at a temperature equal to or higher than the oxidative reaction in reactor II.
4. Process according to at least one of Claims 1 to 3, characterized in that the reaction in step b is conducted with a reaction mixture comprising, as well as alkyl alkoxyisobutyrate, also methacrylic acid, and also dimeric methacrolein (DIMAL) and derivatives of dimeric methacrolein, such as DIMAL esters, and also water and free alcohol.
5. Process according to at least one of Claims 1 to 4, characterized in that the second fraction is reacted in reactor III at a temperature of at least 90°C, preferably of at least 110°C.
6. Process according to at least one of Claims 1 to 5, characterized in that the second fraction is reacted in reactor III in b in the presence of a catalyst, preferably in the presence of a Bronsted acid.
7. Process according to at least one of Claims 1 to 6, characterized in that the reactor output from reactor II is freed of methacrolein and partly of the alcohol in a first distillation column to obtain a stream comprising alkyl methacrylate, water, an alkali metal methacrylate and/or methacrylic acid, alkyl alkoxyisobutyrate and alcohol, which is then admixed with a strong acid and separated in an extraction into a hydrophobic phase comprising alkyl methacrylate, the greater fraction of MAA and alkyl alkoxyisobutyrate, and a hydrophilic phase comprising water, the alcohol, and fractions of alkyl methacrylate and methacrylic acid.
8. Process according to at least one of Claims 1 to 6, characterized in that the reactor output from reactor II is freed of methacrolein and partly of the alcohol in a first distillation column to obtain a stream comprising alkyl methacrylate, water, an alkali metal methacrylate and/or methacrylic acid, alkyl alkoxyisobutyrate and alcohol, which is then separated in a second distillation into a light phase comprising alkyl methacrylate and the alcohol, and a heavy phase comprising water, alkyl alkoxyisobutyrate and methacrylic acid.
9. Process according to at least one of Claims 1 to 8, characterized in that the alcohol is methanol, the alkyl methacrylate is methyl methacrylate (MMA) and the alkyl alkoxyisobutyrate is methyl methoxyisobutyrate (MMIB).
10. Process according to at least one of Claims 1 to 9, characterized in that the reaction in reactor III is effected at a temperature between 80 and 170°C.
11. Process according to Claim 6, characterized in that the Bronsted acid in reactor III is sulfuric acid.
12. Process according to at least one of Claims 1 to 11, characterized in that the first reaction stage in reactor I is a reaction of propanal with formalin.
13. Process according to at least one of Claims 1 to 11, characterized in that the first reaction stage in reactor I is a reaction of isobutene and/or tert-butanol with atmospheric oxygen in the presence of a heterogeneous catalyst at temperatures of 300 to 500°C to form methacrolein, which is condensed and worked up to a purity of at least 80% and isolated in liquid form and is then sent to the further reaction in reactor II of oxidative esterification.
14. Process according to Claim 12, characterized in that the reactor output from reactor II is freed of methacrolein and partly of the alcohol in a first distillation column to obtain a stream comprising alkyl methacrylate, water, an alkali metal methacrylate and/or methacrylic acid, alkyl alkoxyisobutyrate and alcohol, which is then separated in a second distillation into a light phase comprising alkyl methacrylate and the alcohol, and a heavy phase comprising water, alkyl alkoxyisobutyrate, methacrylic acid, dimeric methacrolein and optionally an alkyl ester of dimeric methacrolein.
15. Process according to Claims 6 and 11, characterized in that the dimeric methacrolein is cleaved in reactor III to methacrolein, and the optionally present alkyl ester of dimeric methacrolein to methacrolein and an alkyl methacrylate.
16. Process according to Claims 14 and 15, characterized in that the methacrolein is separated from the alkyl methacrylate in a later distillation stage and returned to reactor II.
17. Process according to Claim 16, characterized in that the reactor output from reactor II is freed of methacrolein and partly of the alcohol in a first distillation column to obtain a stream comprising alkyl methacrylate, water, an alkali metal methacrylate and/or methacrylic acid, alkyl alkoxyisobutyrate and alcohol, which is then separated in a second distillation or an extraction into a light or hydrophobic phase comprising alkyl methacrylate, and a heavy or hydrophilic phase comprising water, alkyl alkoxyisobutyrate, methacrylic acid and terephthalic acid, and in that the terephthalic acid is removed from the reactor output from reactor III as high-boiling component by distillation or as hydrophilic component by extraction.
18. Process according to at least one of Claims 1 to 6, characterized in that the Bronsted acid used is recycled into reactor III or another workup step.

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